Poka-Yoke Basics

Poka-Yoke

Poka-yoke or Mistake-proofing

Poka-yoke (poh-kah yoh-keh) is a Japanese word used in manufacturing environment for “mistake-proofing” or avoiding (yokeru) inadvertent errors (poka). A poka-yoke is a quality tool or method that makes it more difficult to make mistakes and therefore reduces variability in the process caused by human intervention. In a broader sense, the term can refer to any behaviour-shaping constraint designed into a process to prevent incorrect operation by the user. It is only applicable when human error is the root cause of the defect and is most often used in applications where defects have large and expensive consequences for the end-customer.

History

The term “Poka-Yoke” was developed by Shigeo Shingo in 1960. Initially, it was called “Baka-Yoke” which means “Fool-Proofing” or “Idiot-Proofing” in Japanese. This was taken as too offensive by an operator at the time and therefore it was changed to “Poka-Yoke”.

Sping application poka-yoke

Shingo was and industrial engineer working at Toyota who was assigned to improve a spring application process where workers had to insert two springs into a switch. A defect resulted if a spring was missing. To mistake-proof this process, Shingo san installed a second operation in the process. This operation required the worker to insert first two springs in a holder, like displayed on the picture. From this holder, the springs were inserted sequentially into the switch. If an operator would forget a spring, he would notice this when moving forward to the first operation.

This additional step added time but increased process reliability. The chances for making mistakes and the need for rework reduced. Therefore customer and internal satisfaction increased significantly.

Shingo eventually formalized the concept of Zero Quality Control (ZQC). He says that mistakes and defects go hand in hand. A mistake results in a defect if it is not prevented or detected.

Poka-yoke and Lean Manufacturing/Toyota Production System

TPS House

In Lean Manufacturing or in the Toyota Production System, poka-yoke is often used in the production process to support the operator to prevent defects from occurring.

In the pyramid of TPS (Toyota Production System), poka-yoke is part of the Jidoka and Autonomation pillar. Jidoka, Autonomation or Intelligent Automation covers the scope of machines and systems that stop automatically in case of abnormalities or defects. It also applies to operators that have the responsibility to stop their operation in case of an abnormality, call the team-leader and wait for further instructions (Stop – Call – Wait).

In Lean and TPS, muda or waste, is one of the primary items that needs to be prevented in a production process. Poka-yoke helps to reduce this waste by preventing errors and rework.

Poka-Yoke is an inherent component of the Lean Manufacturing Principles but is also often applied in Six-Sigma projects to reduce variation.

Advantages

Advantages

It is obvious poka-yoke devices have many advantages. A bunch of them were listed below:

They increase quality and delivery performance to your customer and so increase customer satisfaction. Warranty costs are therefore reduced.

Poka-yoke systems are often very cheap. They are one of the most cost-efficient methods to prevent and detect errors in a production process.

They are also very effective as they focus on preventing the defect to occur. This results in minimal waste in the production process. Non-value adding activities are removed. And if prevention is impossible, they assure detection as early as possible to the occurrence of the defect.

They imply a behavior of immediate response and so reduce the need for remedial actions or rework.

Simplicity is key in Poka-Yoke. Poka-yoke systems must be easy to implement and easy to understand.

They implement the first-time-right principle, 1st piece quality is assured.

Poka-Yoke devices leverage teamwork and knowledge of the people. By brainstorming on how to prevent defects, people from multiple divisions are forced to work together and so improve employee satisfaction and ownership.

They make processes insensitive for human mistakes.

They make inspection abundant and so reduce inspection related costs.

They result in easier set-ups and decreased set-up times and so result in increased capacity and flexibility.

They increase safety for operators as errors are prevented.

Disadvantages

Disadvantages

Although we are a huge fan of poka-yoke systems, we need to acknowledge some minor disadvantages.

Poka-yoke devices prevent only human errors, not errors that occur as a result of a poorly designed or programmed machine.

They prevent defects, but this prevention might result in damage on the part, if the poka-yoke is poorly designed. For example the operator might try multiple times to fit the part wrongly and therefore create scratches on it.

It is not easy to convince all stakeholders to implement poka-yoke devices. The practical method how to implement is explained on Implement Poka-Yoke but once you start, you will be on a road of continuous improvement.

Most used

In the manufacturing industry, following poka-yoke devices are most often used.

location pins and guide pins

alarms and error detection systems

limit switches

checklist

counters
Most used poka-yoke

Fun Fact

Toyota has an average of 12 mistake-proofing devices implemented at each work station. They also have a goal of implementing each poka-yoke for a cost of below 150$.

Note: Multiple sources were used for summarizing the content on this page. You can find these sources on the following page: References.